In a shot peening method, shots are projected toward a surface of a work, so that a clean surface is formed by removal of the surface or a compressive residual stress is provided to the surface by plastic deformation thereof. In this manner, the shot peening method is advantageous for improvement in the life-span and the reliability of the work. A direct pressure type device in which shots are injected and carried by a compressive gas, and a suction type device in which shots are included in a gas flow having a pressure difference, are used as a projection device of shot peeing.
The shot peening method is used for a pipe (for example, a hollow stabilizer or a hollow coil spring) having a hole. In a pipe, a wall thereof is thinned in order to reduce the weight thereof. The pipe may not be broken from an outer surface that is typically subjected to shot peening, but it may be broken from an inner surface thereof. Therefore, the inner surface is subjected to shot peening, so that the weight reduction by thinning of wall of pipe and the life-span extension may be simultaneously improved.
In this shot peening method, various special members are used for projecting shots toward an inner surface of a hole of a pipe. For example, in a technique proposed in Shotpeener Summer 2004, a hose is provided as a special member at a shot projection side opening of a nozzle, and it is disposed in an inner surface of a hole of a pipe.
However, in the technique proposed in Shotpeener Summer 2004, the hose cannot pass along the inner surface in the hole, depending on an inner diameter of the hole of the pipe and a shape thereof. In this case, when the inner diameter of the horse is small, projection amount of shots is decreased. Due to this, it is necessary to exchange a hose in accordance with the inner diameter of the hole of the pipe and the shape thereof, so that exchange of setting of shot peening apparatus is necessary. Thus, since time is consumed and cost is increased, the workability and the general versatility are insufficient.
In order to solve the above problem, Japanese Unexamined Patent Application Publication No. H5-138535 has proposed that a reflection member is provided as a special member in an inner surface of a hole of a rod-shaped pipe so as to face an entrance opening of a nozzle. In this technique, the pipe is moved by a moving device along an axial direction of the pipe, so that the reflection member is moved relatively to the inner surface in the hole of the pipe.
However, in the technique proposed in Japanese Unexamined Patent Application Publication No. H5-138535, the overall of the pipe is moved relatively to the nozzle and the reflection member, so that it is difficult to apply this technique to a hole of a pipe having a curved portion.
In stead of a shot peeing method, for example, Japanese Unexamined Patent Application Publication No. 2002-137166 has proposed a technique that a compressive residual stress is provided to an inner surface of a hole of a pipe by using a surface enhancement apparatus. In the surface enhancement apparatus of this technique, a shaft portion and a collision element connected thereto are rotated, and the collision element is collided on the inner surface in the hole of the pipe. However, in consideration of practical use, it is difficult to realize this technique.